1. Field of the Invention
The present invention relates to a piezoelectric element and a manufacturing method thereof, further to an electronic device and an ink jet device including the relevant piezoelectric element.
2. Description of the Related Art
A piezoelectric element exhibits the piezoelectric effect, which produces an electric field due to a strain, and the inverse piezoelectric effect, which produces a strain due to an applied electric field. A piezoelectric element which is mainly formed of lead zirconate titanate (hereinafter, called “PZT”) with addition of an extremely small amount of an element such as represented by strontium, barium or niobium is used as a piezoelectric film.
Conventionally, production of a piezoelectric element has included: mixing a raw material powder for a piezoelectric film, pressurizing the raw material powder to sinter it, machining the sintered material to form a piezoelectric film, and subsequently holding the piezoelectric film between electrode materials to polarize the piezoelectric film, thereby producing piezoelectricity. However, recently, as a device becomes miniaturized, it becomes necessary to mount a piezoelectric element on a thinner, smaller area. A piezoelectric element of an oxide-based substance represented by PZT is a brittle material, and therefore it has a machining limit to be thinned up to about 0.1 mm, so that it can be made thin only up to about 0.1 mm. Further, an effect exerted by an adhesive is not negligible in a high frequency band. Therefore, in order to produce a thinner piezoelectric element without a bonding process, various film formation methods such as a sputter method, a CVD method, an aerosol deposition method, a hydrothermal synthesis method and a sol-gel method, etc. have been devised and a piezoelectric element has been manufactured by way of trial using such a method.
Particularly, the aerosol deposition method has a high rate of film formation and recently, particularly attracts attention as a film formation method. FIG. 1 illustrates a rough outline of an aerosol deposition device. This method is that, first, gas 6 such as air, etc. is supplied into an aerosol formation chamber 1 having material particles such as PZT, etc. put therein, and the material particles are aerosolized. Then, the aerosolized particles are directed to a film formation chamber (film forming chamber) 2 connected by a carrier pipe 3, by a differential pressure between both chambers, the material particles are squirted from a nozzle 4 provided on a front edge of the carrier pipe 3 to a substrate 8 on a stage 5, forming a film on the substrate 8, and an air within the film forming chamber is exhausted by vacuum pump 7.
In addition, there is an ink jet device as one of electronic devices using a piezoelectric element. The piezoelectric element may be used for an ink jet head of the ink jet device. In the ink jet device using the piezoelectric element, a system is known that a pressure is generated by applying a voltage to a pressure generator using the piezoelectric element to discharge ink. The ink jet device of this system includes a pressure generator having laminated layers of a pressure generation member of PZT (lead zirconate titanate) etc., a metal plate and ceramics, ink as a flying medium and a nozzle plate having a ink discharge hole. In the ink jet device having such a pressure generator, heads of various types such as a type using a bending mode called “Kyzer type”, a direct pressing type called “piston type” and a type using a shear mode in which a side wall is moved are manufactured in the market.
When a thin film piezoelectric element is formed, there is a method by which, first, a lower electrode is formed on a substrate and next a piezoelectric film is formed by various film formation methods. Subsequently, processes such as a baking process, a formation process of an upper electrode and a polarization process are carried out. To provide a sufficient piezoelectric performance, a baking process at several hundred degrees Celsius is necessary after film formation of the piezoelectric film. This baking process can facilitate enlargement of a crystal grain size and improvement of crystallization, providing a higher piezoelectric performance. In addition, the higher the baking temperature is, the higher piezoelectric performance tends to develop.
However, baking may present a problem that mutual diffusion among a substrate, an electrode and a piezoelectric film occurs. Specifically, as described in Japanese Patent Application Laid-Open No. H08-274573, when a Si wafer is used as a substrate, Ti and Pt are used as an electrode, and a piezoelectric film is formed of PZT, then Si, Ti and Pt diffuse into the PZT film and Pb diffuses into the Si wafer, though there is difference in the degree of diffusion depending on a baking temperature. As described above, if diffusion occurs, a Pt component drops out of the PZT film and further impurities get into the PZT, which presents a problem that electric characteristics may not be enhanced sufficiently. Also, when Pb diffuses into the Si wafer, the Si wafer gets hardened, causing a problem that accurate machining may not be allowed when the machining is required in a post-process after film formation. Particularly, when an ink jet head is formed, the backside of a Si wafer on which a piezoelectric film is formed is often cut to form a flow path, and at this time, machining is difficult due to diffusion of the Pb component into the Si wafer.
Moreover, diffusion of a material for a lower electrode into PZT progresses, a part of the material reaches the upper portion of the PZT film, and then the PZT film may not often function as a piezoelectric element because of contacting electrically with an upper electrode formed after baking. As a film thickness of the PZT film becomes thinner, this tendency appears more prominently.
Further, there is a problem that a higher baking temperature oxidizes an electrode itself to provide resistance, so that the electrode may not work as an electrode. For example, to provide adherence between a substrate and Pt, use of Ta, etc. as an adhesion layer has an effect of adherence (see Japanese Patent Application Laid-Open No. 2001-152360), but tantalum oxide may be created even at a comparatively low temperature of about 600° C. Also, it is proposed that an electroconductive oxide film represented by SrRuO3 be used as an electrode (see Japanese Patent Application Laid-Open No. 2000-328223 and Japanese Patent Application Laid-Open No. 2002-016229), but specific resistance starts to increase at about 700° C., then, beyond 800° C., the electrode becomes approximately similar to an insulating film, so not preferable for an electrode.